The semiconductor industry is characterized by a trend toward fabricating larger and more complex functions on a semiconductor chip. The larger and more complex functions are achieved by reducing device sizes and spacing and by reducing the junction depth of regions formed in the semiconductor substrate. The shallow junctions are extremely sensitive to high temperature processing and therefore manufacturers have sought to develop low temperature processes to avoid diffusing dopants in the junction regions. One major area of high temperature processing in a semiconductor fabrication process is thin film deposition. Typically thin films are formed at a high temperature in chemical vapor deposition (CVD) systems where temperatures can reach in excess of 900.degree. C. To avoid exposure of semiconductor device substrates having shallow junctions to such high temperatures several low temperature deposition methods have been developed.
Progress in CVD technology has brought about low temperature metal-organic CVD (LTMOCVD) as one approach to fulfilling the needs for deposition of materials such as Cu, Ti and W. This method can be used to deposit a metal film in the temperature range of about 100.degree. to 350.degree. C. To be successful, this method requires that the organic precursors have a vapor pressure sufficient to allow reliable delivery into a reaction chamber and not condense to a solid or liquid in the delivery system. The selection of precursors having acceptably high vapor pressures has lead to an extensive effort to synthesize organic precursors having the required physical properties. This is especially difficult since most of the compounds having the structure and chemical reactivity necessary to deposit a thin film in an LTMOCVD system are solids at room temperature. Precursors which are solids at room temperature are undesirable because of their tendency, once vaporized, to condense on the surfaces of delivery system before reaching the reaction chamber. The difficult synthesis of LTMOCVD precursors has limited the number of different precursors available for use; and of course, the limited range of available precursors has been accompanied by a concomitant reduction in the range of film types which can be realized by this technique.
Recognizing the limitations inherent in LTMOCVD, liquid phase deposition (LPD) techniques have been developed as an alternative method for low temperature film deposition. One existing LPD method used for dielectric film formation employs sol-gel technology. Using sol-gel technology, a precursor, typically an alkoxide, is dissolved in the parent alcohol and then hydrolyzed under controlled conditions to produce a gel containing the hydrated metal oxide. The gel is then spun onto the substrate surface and dryed leaving a glassy dielectric material on the substrate surface. A fundamental requirement of this technique is the precursor must be highly chemically reactive with water in order to form the gel.
The requirement that the precursor readily undergo hydrolysis is also characteristic of existing precipitation methods. In one precipitation method, hydrofluorosilicic acid is reacted with water to form a supersaturated solution in which silica particles are dissolved. Boric acid is continuously added to maintain a supersaturated solution and SiO.sub.2 is precipitated out of solution at about room temperature. This technique requires that two compounds of silicon be used to form the SiO.sub.2 layer.
To a large extent, the existing LPD methods have not solved the problems inherent with LTMOCVD. For the most part, the existing LPD techniques, such as sol-gel technology and precipitation methods, are limited to the formation of a relatively narrow group of film types. The limited film formation ability of existing LPD techniques relates, in part, to the requirement that the precursors have a specific chemical reactivity, and therefore, does not overcome the primary limitation of LTMOCVD. Accordingly, the development of an LPD technique which places few chemical reactivity requirements upon the precursors and which is capable of reliably forming of a large variety of film types is necessary to meet semiconductor manufacturing needs.